A blow-by gas (including an unburned fuel and/or an exhaust gas) leaked from gap between a cylinder and a piston of an engine to a crankcase is mixed with a misty engine oil scattering in the crankcase, and the engine oil in an oil pan becomes contaminated. Thus, a portion of a cylinder head cover communicating with the crankcase and an intake passage of the engine are communicated with each other via a hose, so that the blow-by gas in the crankcase, which contains oil mist, is actively ventilated by aspiration with negative pressure of the intake passage. This mechanism is referred to as positive crankcase ventilation (abbreviated as PCV), and the hose is referred to as a PCV hose (or a blow-by hose).
Accordingly, the blow-by gas containing oil mist passes through the PCV hose, and the PCV hose is also exposed to heat, and thus the PCV hose is required to be resistant to both oil and heat. In addition, the PCV hose is required to be resistant to ozone in order to avoid deterioration caused by ozone contained in external air. These performances are similarly required of air hoses for automobiles other than the PCV hose.
Thus, many of air hoses for automobiles have two-layered structures, and the two-layered structure is composed of an inner tube made of a butadiene-acrylonitrile copolymer rubber (NBR) that is a typical oil-resistant rubber and an outer cover made of a chlorosulfonated polyethylene rubber (CSM) that is excellently ozone-resistant (Patent Document 1).
However, heat resistance, chemical resistance, and the like of NBR is limited, because the main chain of NBR contains an unstable unsaturated bond (a carbon-carbon double bond), and thus the conventional NBR is not sufficiently resistant to heat when the NBR is used for air hoses that are required to be more highly resistant to heat.
Therefore, hydrogenated NBR (HNBR), in which an unsaturated bond portion is changed to a saturated bond by hydrogenating the unsaturated bond portion so as to improve heat resistance and the like, has been used. That is, an inner tube has a two-layered structure composed of an inner tube made of HNBR and an outer cover made of CSM; or has a three-layered structure composed of an inner tube 51 made of HNBR, a middle layer 52 made of NBR, and an outer cover 53, as shown in FIG. 2.
However, the cost of HNBR is higher than that of NBR, and thus it has been required to improve heat resistance of NBR rather than using HNBR.
As a method for improving heat resistance of NBR, using dibenzothiazyl disulfide as a vulcanization accelerator (Patent Document 2) has been known. In order to enable vulcanization at a high temperature in a short time, using dibenzothiazyl disulfide and/or tetramethylthiuram monosulfide (Patent Document 3) has been also known.
However, according to studies conducted by the inventors of the present invention, these methods did not provide sufficient heat resistance to NBR.